The Sanger-Bredt (Brecht?) Silverbird, a.k.a. "Antipodal bomber", was to fly around the world from an initial speed of 13,000 mph by "skipping" off the atmosphere in a series of progressively lower skips, descending from space to as low as 100,000 feet before ascending again. The greatest technical hurdle would be the tremendous heat loads, as a Sanger vehicle would still be doing over 10,000 mph at the same alitude where the X-15 was reaching 4,000 mph.